D AY 26: P OLYMERS Review: Polymers vs. Metals Strength factors: What factors make polymers strong? 1. Crystallinity 2. Increase the chain length Here’s another one: 3. Decrease chain flexibility. I.e. make the backbone more rigid. Here’s an example.

A NOTHER C OMMON P OLYMER : P OLY S TYRENE Basic Mer Phenyl group The second most commonly encountered polymer. This is a hard, yet cheap commodity plastic. Yes, it can be foamed. This is a thermoplastic polymer. PolymerdensityUTS ksi%ELE ksi PS MDPE

U SES OF PS Cheap, hard commodity plastic. Or foam.

W HY PS IS STRONGER AND LESS DUCTILE THAN PE Crystallinity is less. The bulky side groups interfere with the formation of cystals. BUT The difference is that the backbone is much more rigid. The backbone can’t rotate as easily because of the phenyl groups. Consequently sliding of adjacent chains is inhibited. Strength is increased at the cost of ductility.

T WO T YPES OF PS

T ACTICITY, ACTUALLY THERE ARE THREE

S OME P ROPERTIES OF A S YNDIOTACTIC PS Density: 1.11 g/cc UTS: 10.5 Ksi %EL 1.8% Modulus of Elasticity: 700 Ksi Just a few remarks about the difference: 1. Bulky side groups such as phenyl inhibit crystallinity. 2. Sydndiotactic, ie regular placement on alt. sides promotes crystallinity. 3. Crystallinity enhances strength and stiffness.

I MPROVED PS – H IGH I MPACT PS (HIPS) We form what is called a “graft copolymer.” This is kind of like an alloy. Polybutadiene rubber chain Atactic polystyrene HIPS is strong and tough!!

P OLYPROPYLENE, PP Here’s another simple mer. Look’s kind of like PS except that instead of a phenyl side group, we have a methyl side group. PolymerdensityUTS ksi%ELE ksi PP PS MDPE Side group is not as bulky as the one in PS.

C RYSTALLINITY AND PP We have either considerable crystallinity or none depending on tacticity. Irregular side group placement: amorphous Regular side group placement: up to 60% crystallinity

PP F IBERS The isotactic form, which has high crystallinity, can be turned into fiber. The crystallinity is associated with chain alignment in the longitudinal direction of the fiber. Though PE is more crystalline, it is easier to get PP to form these fibers. Yes, PE fibers are great. (Spectra) But not as economically viable as PP fibers. Nylon, which will be discussed soon is well known for forming fibers.Spectra Lots of PP fiber used in clothing. I.e. mountain climbing thermal underwear.

PP A DVANTAGES 1. Easy to color 2. Dishwasher safe, compared to PE. 3. High strength to weight 4. Doesn’t absorb water readily 5. Can be readily used as a plastic as well as a fiber. See specimens.